Processes are known for manufacture of latex foam resting articles, such as for instance mattresses, mattress parts, cushions and the like.
The above mentioned articles manufacture is carried out both by single moulds and by unlimited length moulds such as a belt or a cart or plate series continuously moving along an annular configuration.
Such continuous processes are used and described in the assignee patents EP-B-0.380.963, U.S. Pat. No. 5,229,138, U.S. Pat. No. 6,086,802, EP-A-1.361.033.
A single mould generally comprises a hollow lower part and an upper part forming the cover, the former and the latter or both are also provided with protuberances for recesses formation.
As it is known, initially a latex and its components comprising mixture is prepared.
Latex is an elastomer polymer dispersion in an aqueous serum.
Herein following the term “components” means all those substances, materials, ingredients and/or fillers whose presence is thought being useful to carry out optimally latex foam article manufacture steps with determined and desired characteristics.
Generally said “components” include surfactants, thickening agents, gelation agents, stabilizers, process accelerators, vulcanization agents, fillers, and anti-oxidizers.
The process employs a tank within which various components are added to a synthetic or natural latex mixture.
The raw components prior to their addition to latex are transformed into an aqueous dispersion using grinders to reduce particles to a few microns size.
Among the most significant components are the surface active and gelation agents.
As it is known, surfactants determine the presence around each single latex particle of a protective barrier opposing their approach; such barrier being formed by electric charges all of the same sign for all latex particles such that they repel each other avoiding a premature and undesired particle coagulation.
Gelation agents have the task of breaking the abovementioned barrier such that various latex particles, no longer subject to repulsion forces may come near or away from each other being subject to vibrations increasing with temperature increase.
During such alternating motion latex particles touch and unite each other carrying out latex coagulation.
Referring back to the mould manufacture process, to the latex and its components comprising mixture in the container a pressurized gas is added and the mixture is agitated at high speed until its foaming is obtained.
A determined solution of a gelation agent destined to carry out the gelating step is added to the latex foam.
At this time, to further clarify the present invention field of application we recall briefly herein that latex foam resting articles have characteristics and process steps different from other foam articles, for instance polyurethane foam.
As far as the process steps one should remember that latex article foaming is achieved inputting an external agent such as pressurized air in the mixture.
In contrast, polyurethane mixture foaming is achieved by direct contact among polyol and isocyanate, that are the same base materials making up polyurethane.
Further latex foam articles have a structure provided with tiny open and uniformly distributed cells for all material thickness, polyurethane foam articles have closed cells except specific polyurethanes in which cells are blown up after specific treatments.
A latex foam article manufacture envisages filling up the mould with a mixture comprising latex and components already subjected to foaming.
It is to be noted that after foaming the latex mixture is very unstable such that a risk exists of a collapse of cell walls.
With the aim of dealing with such risk the gelating step is carried out.
Gelating may be defined as a homogeneous coagulation in that part of foam aqueous serum is embedded among rubber polymer particles.
Chemically gelating is the transition from “sol” phase to “gel” phase.
The gelating step avoids latex foam collapse after deposit.
The following vulcanization step is carried out to determine the highest elasticity degree in the final article that is the elastic deformation, density and uniformity article characteristics that will assure comfort conditions for the user in a rest state.
During the vulcanization step the mould is heated going through a tunnel within which steam is supplied.
At the end of the vulcanization step the mould upper part is rotated or it is lifted with respect to the hollow lower part to allow for article extraction.
In view of the above, it is to be noted that resting articles may be made up by many layers to each of which a specific, density, hardness or mechanical or chemical strength to provide specific performances, characteristic is attributed.
Generally such articles, particularly mattresses, are assembled joining various layers one to another using glues or adhesives.
Unfortunately such technique is un-satisfactory.
As a matter of fact gluing materials are toxic and anyway do not guarantee a lasting attachment between layers.
It may be envisaged to realize multiple layers articles by pouring in an open mould a first latex foam followed by a second latex foam with different characteristics from the first one.
Anyway in such case, because the two mixtures are substantially liquid, one would have a remixing of the two mixture with ensuing variation of the two layers preestablished geometry.
The final article would not benefit from layers separate one from the other by a neat boundary surface.
Therefore one would obtain an article with a fuzzy physical or chemical characteristic and with ensuing unwanted performance.
In case of mould with a cover it is to be noted that the foam material filling up in a foam is as a rule carried out with excess material relative to available volume in the mould cavity.
Therefore during the cover closing the upper mixture would be compressed against the lower mixture with the two mixtures obvious un-homogeneous distribution.
One may envisage using a multi-layered latex foam mattress continuous manufacture process.
Anyway even in such case injecting the second mixture onto the first one at a laying station, would not provide results different from the above mentioned ones for single open moulds.